FIG. 1 schematically shows prior art image recording apparatus 100 with light-emitting diode (LED) printbar 101. Printbar 101 is an example of an LED full width array imager. An LED full width array imager consists of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor in a process direction, and by selectively energizing the LEDs at the proper times in a scan direction, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
The following provides further detail regarding prior art apparatus 100. Printbar 101 includes: LED's controlled according to recording signals supplied from an unrepresented external device; a rotary drum 102 provided with a photoreceptor along the periphery thereof; a rod lens array 103 for focusing the light beams of the LED's in the printing head 101 onto the photoreceptor surface of the drum 102; a corona charger 104 for charging the photoreceptor in advance; a developing station 105 for developing an electrostatic latent image with toner; a recording sheet 106; a cassette 107 housing a plurality of recording sheets 106; a feed roller 108 for feeding the recording sheet 106 from the cassette 107; registration rollers 109 for matching the front end of the recording sheet with the leading end of the image formed on the drum 102; a transfer charger 110 for transferring the developed image from the drum 102 onto the recording sheet 106; a separating roller 111 for separating the recording sheet from the drum 102; a belt 112 for transporting the recording sheet; fixing rollers 113; discharge rollers 114 for discharging the recording sheet onto a tray 115; a blade cleaner 116 for removing the toner remaining on the drum 102; a container 117 for the recovered toner; and a lamp 118 for eliminating charge remaining on the drum 102.
FIG. 2 shows typical curves for LED degradation with time. Curves 150 show performance at rated current. Curves 152, at 2.5 times rated current, show accelerated life tests. It should be noted that during early life most LEDs increase in power as they “anneal” and then exponentially drop in time. Thus, for individual LED emitters, for example, spaced at 600 dots per inch (dpi) or 1200 dpi spacing in a full process width printbar, such as printbar 101, power will vary with time with an increase initially followed by a long slow degradation. It is commonly understood that the degradation is mainly dependent on the total usage (time×duty cycle) of the emitters and current level, with some variation from batch to batch due to semiconductor processing.
FIG. 3 shows typical LED usage for LED printbar 101 with respect to portions of the photoreceptor for drum 102. The total usage time for LEDs in a printbar can vary substantially, for example, depending on the portion of the photoreceptor with which an LED is aligned in a process direction. For example, LEDs aligned with portion 154 of the photoreceptor outside the paper margin with may only have a 0-1% duty cycle, and LEDs aligned with portion 156 of the photoreceptor inside the paper margin may have a 5-20% duty cycle. Thus, after the equivalent of 500 hrs 100% duty cycle for LEDs aligned with portion 156, LEDs aligned with portion 154 may have only 50 hrs or less of 100% duty cycle. As a result, LEDs aligned with portion 154 may be 5-10% brighter, as seen in FIG. 2, than at time 0 and the rest of the LEDs may have degraded 10%. As an example, if a full page width half-tone image is printed without any margins, thus using LEDs aligned with portions 154 and 156, there may be a noticeable band at the location of the border between portions 154 and 156 due to the 15-20% difference in power level noted above between LEDs associated with portions 154 and 156.
U.S. Pat. No. 4,982,203 discloses correcting light output for LEDs in an LED printbar by predicting and measuring light degradation and then modifying current to the LEDs accordingly. U.S. Pat. No. 5,016,027 teaches periodic calibration and adjustment of on-time for LEDs in an LED printbar using manual scanner calibrations or with optical sensor feedback. U.S. Pat. No. 5,668,587 discloses determining on-time differences among LEDs in an LED printbar and uses an average aging curve to determine adjustments to a drive circuit.